I just bought a on one 650b frame, lovely bit of kit. However, it needs a zs44 headset, which is something I've not come across yet. But, it gives me the option for straight or tapered? Now I know what a tapered fork is, question is, is it dependent on the frame as to weather you have to fit taper or straight? With such a large headstock, I don't wanna buy a straight fork and headset only to find its no good. Frames aren't taper/straight dependent are they? I can't find a straight answer anywhere...

With the 44mm head tube, you can use a straight or tapered fork, but they'll require different headsets. Zero stack headset(also called semi-integrated) for the straight steerer. For a tapered fork you have to use an external lower cup to fit the taper(which will raise the front of the bike and slacken the head tube angle a bit.)

they can be, depends on the frame, although you can get adapters to work round this.

Research - Park Tool Co. » ParkTool Blog » Headset Standards

well I'm edging towards a standard straight taper, so I assume that this headset would be right?

Cane Creek 40-Series ZS44 ZeroStack Headset | Chain Reaction Cycles

And i guess the 8mm/15mm cover size option is only how high the top cup protrudes?

£35.99 at Wiggle, both top cap options available Wiggle | Cane Creek 40 Series ZeroStack 1 1/8 Inch Headset | Headsets

Your fork will determine if you need tapered or not, quick look at CRC indicates there aren't any non tapered 650b forks, so you will need to look at the tapered headset options( i.e. not the ZS44).

Tapered forks aren't the future, they are the now, if buying new today, can't think of a single reason not to get tapered.

Knowing which frame you have would be useful, saying an On-One is too vague, too many different models & specs.

I've got this frame -
On-One 45650B Frame | Planet X

and I fancy these forks

MKM 35 Rigid Carbon MTB Fork | Planet X

I know its 650B but with no bridge and it makes up about 80mm travel, it'll accompany a 650B wheel into the 26" forks I would've thought?

The ZS44 will work fine, to run a tapered fork, you would need adapter bottom cup, like this Cane Creek 40-Series EC44-40 Bottom Assembly | Chain Reaction Cycles

For the fork, looks OK, with rigid, the tire you are using will have quite a bit of impact on the height change as well, so wouldn't get too hung up on what they say it's spec'd for.

I won't be running tapered if I don't have to, so that's a big help. Thank you very much, you've got me out a right pickle.